A speeding vehicle on the American side of a U.S.-Canada border crossing near Niagara Falls blew up Wednesday, killing two people and causing federal authorities to shut down four border checkpoints amid international concern.

The two people found dead were inside the vehicle that exploded. The FBI's field office in Buffalo said it is investigating the explosion on the Rainbow Bridge, which connects the two countries across the Niagara River.

But New York Gov. Kathy Hochul said that, based on a preliminary investigation, there is no sign of terrorist involvement. Law enforcement was not aware of any threats to the area and the investigation is ongoing, she said. The vehicle was incinerated and pieces of the car are widely scattered, she said.

"The world is watching to find out what happened here," she said. "Based on what we know at this moment, and again anything can change, there is no sign of terrorist activity."

"The FBI is coordinating with our local, state and federal law enforcement partners in this investigation," a statement posted on X reads. "As this situation is very fluid, that's all we can say at the time."

New York Gov. Kathy Hochul said that, based on a preliminary investigation, there is no sign of terrorist involvement. Law enforcement was not aware of any threats to the area and the investigation is ongoing, she said. The vehicle was incinerated and pieces of the car are widely scattered, she said.

"The world is watching to find out what happened here," she said. "Based on what we know at this moment, and again anything can change, there is no sign of terrorist activity."

A witness, Mike Guenther, told WGRZ-TV that he saw a vehicle speeding toward the crossing from the U.S. side of the border when it swerved to avoid another car, crashed into a fence and exploded.

“All of a sudden he went up in the air and then it was a ball of fire like 30 or 40 feet high,” Guenther told the station. “I never saw anything like it.”

President Joe Biden and Attorney General Merrick Garland have been briefed on the explosion. Biden and his team are closely following developments, according to The White House.

The Federal Aviation Administration initially ordered a halt to international arriving and departing international flights at Buffalo Niagara International Airport on Wednesday afternoon in response to the potential threat, and lifted it after about two hours, CNN reported.

The Canada Border Services Agency in a statement on X said it is "aware of an evolving situation at Rainbow Bridge."

"We are liaising with our U.S. counterparts on this matter. The FBI is leading on the ongoing investigation," the statement reads.

On Wednesday afternoon, New York Assemblymember Michael Novakhov wrote on Facebook "all border crossings between the U.S. and Canada remain closed and local government offices in the immediate area have been closed and evacuated."

New York City Mayor Eric Adams said his team and NYPD "have been closely monitoring the situation on the ground in Buffalo after an explosion at the Rainbow Bridge, and we've already sent NYPD officers upstate to support efforts on the ground."

What happened at the bridge crossing?

Photos and video taken by news organizations and posted on social media showed a security booth that had been singed by flames. Gov. Hochul said the driver of the car was traveling at an "extraordinarily high rate of speed" before it crashed into the median and went airborne. A Western New York resident was involved, she said.

The explosion left one border patrol agent who was working in the booth injured, Hochul said.

American and Canadian airlines increase security, halt flights; Amtrak suspends routes

International flights were cancelled at Buffalo Niagara International Airport. Southwest Airlines, which has 14 daily departures to six cities from Buffalo’s airport, issued a travel advisory for customers who wanted to alter their Wednesday flights because of potential disruptions, spokesperson Chris Perry said.

The Niagara Frontier Transportation Authority released a statement on Wednesday afternoon to inform travelers that while Buffalo and Niagara Falls Airports are "fully operational," there will be extra security measures.

"In the response to the incident that is unfolding at the Rainbow Bridge, the NFTA is increasing security system-wide," a statement from on X reads. "Cars coming into the Buffalo Airport will undergo security checks and travelers can expect additional screenings."

They advised travelers "to give themselves time for these extra precautions in addition to holiday travel."

What is the Rainbow Bridge?

The Niagara Falls International Rainbow Bridge is an arch bridge that connects the cities of Niagara Falls, New York, in the United States and Niagara Falls, Ontario, in Canada. The bridge can be crossed by car, on foot or by bicycle.

Contributing: Bart Jansen, Michael Collins, Dinah Pulver